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Referencing ground conditions

Mellors, T.

October 31, 1995

London, UK

World Tunnelling V.8 No.9, November 1995

It should not need to be said that the ground plays an overriding influence on tunnelling projects and underground works. However, we evidently need to be continually reminded of this truism, not only for the planning of such projects, but also to provide the framework for design and construction activities, including contract procurement and execution. Even a cursory glance through recent tunnelling related publications reveals numerous instances of situations where either 'unforeseen' ground conditions or the nature of the ground, directly or indirectly, have led to problems or difficulties during construction.

The development of more sophisticated soft ground and hard rock tunnelling boring equipment and systems which preclude ready and direct access to the face in the normal course of operations effectively requires a higher level of 'certainty' in relation to the ground to be encountered. Whilst such equipment and systems may be capable of dealing with wider ranges of materials and mixed-face conditions, their overall success is still dependent on an accurate assessment of the ground to be encountered and its variability. It is all very well having the facility to probe ahead of the face but of little comfort if conditions are found which will present problems or difficulties that have not been allowed for with the installed equipment.

Similarly, the increasing use of 'Observational Methods' in tunnelling (such as NATM) necessitates a fundamental appreciation of the ground to be encountered and its likely response to excavation.

The CIRIA, Report 79 'Tunnelling- improved contract practices' noted that, in relation to tunnelling, ‘the ground is the principal construction material, supplied by nature and seldom to specifications that engineers would choose". Because of the role and significance of ground in underground construction, there will always be a need to obtain reliable and realistic information for planning, design and construction purposes and for limiting risks to promoters and constructors.

GROUND INVESTIGATION

From the outset it should be acknowledged that the implementation of any ground investigation programme must be regarded as an iterative process requiring a flexible approach in terms of both techniques and contract so that the nature and scope of works can be varied in accordance with the conditions being encountered in relation to the proposed project. A phased approach to investigation, allowing a proper and full assessment of all the data collected at the end of each phase and thus permitting appropriate planning of subsequent phases, is essential for the optimisation of the investigation programme. In the most basic form, this might involve merely a desk study followed by a single programme of drilling, boring, sampling and testing. This procedure could apply, for example, in the case of a relatively simple project of limited extent where there is already available reliable information from previous investigations. At the other extreme. on a more complex project in a location where there is little precedent experience and background information, the procedure may entail several programmes of drilling/boring, sampling and testing as the scheme advances through feasibility, preliminary design and final design stages. The purpose of the various phases in such instances may change from exploratory to confirmatory in respect of the information being sought. The findings from each phase could impinge on the overall planning and design of the project necessitating amendments/modifications to the ground investigation programme.

Similarly, scheme arrangements or alignments can change during planning and design stages for technical, political and environmental reasons, which have an immediate consequence on the scope of the investigation programme. However, from experience it is not uncommon to find that the implications of such changes in relation to ground conditions are not adequately investigated, usually on the basis of cost and/or programme implications, with consequent problems ensuing during construction.

Acknowledging the purpose of investigation programmes, it can be readily appreciated that, inevitably, it is not always possible to define with certainty at the outset what the final scope and extent, and hence cost, of an investigation programme may be. Preconceptions at the initial stages must be avoided to ensure flexibility in the investigation scope and there can be no justification whatsoever for limiting the scope of works on cost grounds alone. These requirements immediately raise difficulties for the promoter of the scheme as he cannot be assured at the outset of the out-turn cost for site investigation works. However, it must be a matter of education to explain that monies invested appropriately at the beginning of the project will reap untold benefits later. It is far better for the promoter to have an accurate estimate of the out-turn cost of the project overall, based on reliable and realistic information relating to the principle construction material, rather than just the ground investigation works - design changes during construction necessitated by ground conditions invariably carry a cost premium.

RESIDUAL RISK

At the end of any investigation programme, there will always be a degree of ‘residual risk’ associated with the ground in terms of the possibility of subsequently encountering 'unforeseen' conditions. The degree of this risk is a function not only of the pertaining geological environment and the extent of the investigation works, but also the interpretation of the conditions. Bearing these in mind and in particular the vagaries of Mother Nature, it has to be accepted that this residual risk can never be eliminated - it can be reduced by additional work but only to an unknown degree prior to the project commencing. It is when the project is complete that the overall adequacy of the investigation programme (including interpretation) can be judged.

Limiting the degree of residual risk prior to construction can be achieved by extending the scope of the investigation but there comes a point when the cost effectiveness of further work - in terms of spend over return - diminishes to such a point that it can no longer be justified. One would like to believe at this point, that the investigation has fulfilled the basic objective of ensuring that ‘economical design and construction by reducing to an acceptable level the uncertainties and risk that the ground poses to the project’. This requires extreme foresight, experience and judgement (as well as a highly polished crystal ball) since it requires knowledge of what constitutes 'uncertainties' in order to bring them to an acceptable level.

TECHNIQUES

Over the years, the geotechnical community has worn the sackcloth and ashes heaped on it with fortitude. It has long been aware, however, that the problems experienced with the ground were not all of its own making but stemmed, inter alia, from a lack of education and understanding by others of the importance of ground investigation and more fundamentally of geotechnical engineering and engineering geology. To be asked by a non- geotechnical engineer "What's a phi for sand?" and in response to the query "dense or loose" to be then told "If you are going to make such a meal of it, I will look it up myself- does little to foster the spirit of co-operation so necessary between the disciplines in civil engineering.

The significant amount of technical literature, in the form of papers and conference proceedings, relating to the investigation and testing of the ground stands as a testimony to the endeavours of the geotechnical community worldwide in seeking to improve standards and techniques. The accreditation of drillers and testing laboratories in the UK, at least, also serves to demonstrate a willingness and intent to provide quality services in these respects. Notwithstanding such endeavours, in 1991 the Institution of Civil Engineers (ICE) published ‘Inadequate Site Investigation’ after having produced a 'Specification for Ground Investigation' in 1989, prompted by reports from the National Economic Development Office, the National Audit Office and the Transport. Research Laboratory which attributed significant delays and additional costs on various types of projects to problems associated with the ground. The document contained the recommendation that the ICE should establish a broadly based steering group to carry out a number of tasks, which included the development of a national specification for site and ground investigations, and the development of quality management systems for site and ground investigations.

Accordingly, a Site Investigation Steering Group (SISG), with representation from 21 organisations, was established which produced four documents under the overall heading of ‘Site Investigation in Construction’. Included in the series was another new 'Specification for ground investigation', a document entitled 'Without site investigation ground is a hazard' (although for the reasons given earlier, it might be said that even with site investigation ground is a hazard) and a document relating to the planning, procurement and quality management of site investigations. Together these documents provide a comprehensive framework for the procurement of site investigation services and a specification for the works that allows for additions or modifications to suit a particular programme, recognising the need for flexibility.

The SISG concluded that ‘Inadequate site investigation can arise from a lack of awareness of the importance of ground, inadequate amount or focus of finance, insufficient time and a lack of geotechnical expertis’. It also noted that additional delays and costs, often far in excess of the price of the original site investigation, stemmed basically from poor planning, poor implementation, poor communication and poor interpretation.

In seeking to ensure that site investigations can be planned, procured, interpreted and validated dated appropriately, the Site Investigation Steering Group identified the key positions of 'Client's Principal Technical Adviser' and 'Geotechnical Adviser' for the individual who should be responsible for the supervision and interpretation of a site investigation. The Group even went so far as to prescribe guidelines for the qualifications and experience necessary to fulfil the role of Geotechnical Adviser as well as guidelines for the qualifications and experience for other geotechnical personnel who were likely to be involved with site investigation works.

By adopting the measures and procedures set out in the documents, the perceived problems of poor planning, poor implementation and poor communication should be minimised with the promoter being made fully aware of the purpose of, and need for, the works at all times. The Recording and Reporting of Ground Conditions Drill hole and borehole logs inevitably form part of Contract documentation. Geologists and engineers involved in their preparation are only too aware that what they write on the logs will inevitably be used against them.

CLARITY IS VITAL

The logging of materials and conditions encountered during an investigation programme is of necessity a subjective exercise and there are various internationally recognised guidelines and standards which seek to limit this subjectiveness and rationalise the procedures for the description of materials. In the U.K., the most widely accepted form is to follow BS 5930. It is not uncommon, however, to find in factual reports of site investigation, that is those reports containing logs, results of laboratory and field testing and so forth, that the logging was carried out 'generally in accordance with BS 5930' or 'using the principles given in BS 5930'. Whilst seeking to be helpful by referencing the basis on which the logs had been prepared, they do imply that there may have been some departure from the standard or that there may have been augmentation or supplementation of the standard. Without clear definition and clarification of all the terminology used in presenting factual data, the data base is immediately open to interpretation and argument. There can be no excuse for not defining the precise meaning of any terminology used in presenting factual data, as shown for example in Figure 1, to avoid the sort of problem than can readily occur in describing recovered material as illustrated in Figure 2. Such definition can save much discussion, debate and argument later.

The situation to be avoided at all costs is that so eloquently summed up in the quote purported to come from Watergate - "I know you believe you understand what you think I said, but I am not sure you realise that what you heard is not what I meant". A Tenderer should be left in no doubt as to the basis and meaning of the terms used in the information provided.

CIRIA 79 advocated the use of 'Reference Ground Conditions' at the time of Tender for tunnelling works as a means for allocating and reducing risk. Such Reference Conditions would clearly define the ground condition to be anticipated in the works and would provide the basis against which the encountered ground conditions would be monitored for variation. They would also create a 'level playing field'. This laudable methodology, however, is still not necessarily applied today and all too often acrimonious disputes arise over purported 'unforeseen conditions' which could have been obviated.

In developing Reference Ground Conditions, it is necessary to undertake interpretation of all the data and present it in a form appropriate to the project and to the form of contract. It is commonly assumed that interpretation will be carried out for design purposes in any event as well as for identifying the residual risks as part of an overall project risk assessment. There is, however, obvious reluctance on occasion to part with this information. Contractors are still asked to prepare their Tenders on factual data only: borehole logs, laboratory and field testing data. When one considers the latitude afforded by lack of definition in terminology, as referred to earlier, as well as that arising from a Tenderer's own assessment of the information, there is little wonder that this approach is a potential recipe for disaster. It does not seem too long ago that, even with this arrangement, disclaimers were, on occasions, attached to the factual data provided to the Tenderer. Quite what they were then expected to base their Tenders on remains open to speculation.

Furthermore, in not seeking clarification from the Tenderer as to what they based their Tender on, adjudication of Tenders becomes something of a lottery if, as frequently stated, the lowest Tenderer will not necessarily be appointed.

CIRIA 79 advocated the use of 'Reference Ground Conditions' at the time of Tender for tunnelling works as a means for allocating and reducing risk. Such Reference Conditions would clearly define the ground condition to be anticipated in the works and would provide the basis against which the encountered ground conditions would be monitored for variation. They would also create a 'level playing field'. This laudable methodology, however, is still not necessarily applied today and all too often acrimonious disputes arise over purported 'unforeseen conditions' which could have been obviated.

In developing Reference Ground Conditions, it is necessary to undertake interpretation of all the data and present it in a form appropriate to the project and to the form of contract. It is commonly assumed that interpretation will be carried out for design purposes in any event as well as for identifying the residual risks as part of an overall project risk assessment. There is, however, obvious reluctance on occasion to part with this information. Contractors are still asked to prepare their Tenders on factual data only: borehole logs, laboratory and field testing data. When one considers the latitude afforded by lack of definition in terminology, as referred to earlier, as well as that arising from a Tenderer's own assessment of the information, there is little wonder that this approach is a potential recipe for disaster. It does not seem too long ago that, even with this arrangement, disclaimers were, on occasions, attached to the factual data provided to the Tenderer. Quite what they were then expected to base their Tenders on remains open to speculation.

Furthermore, in not seeking clarification from the Tenderer as to what they based their Tender on, adjudication of Tenders becomes something of a lottery if, as frequently stated, the lowest Tenderer will not necessarily be appointed.

INTERPRETATIVE REPORT

It is becoming more common for Tenderers to be provided with an 'Interpretative Report' which provides the ideal medium to set out the Reference Ground Conditions. This document should, inter alia, provide a summary of the engineering characteristics and properties of the materials to be encountered and an interpretation of the ground (and groundwater) conditions in respect of their influence and effect on the proposed works. Such interpretation, of necessity, involves exercising judgement and opinion and should only be undertaken by personnel appropriately experienced, that is 'the Geotechnical Adviser.

Interpretation is more of a subjective exercise than the logging and description of materials referred to previously, since there could very well be more than one interpretation of the available facts. For example, there could be more than one reasonable and acceptable interpretation for the basic distribution and disposition of materials, bearing in mind the necessity to extrapolate between drill holes which are effectively point sources of sampled data in relation to the overall volume of material.

For an Interpretative Report to be useful to Tenderers, it must be decisive. Language which is implicitly or explicitly imprecise should be avoided whenever and wherever possible. Even the use of the word 'may' in relation to an event or situation should be avoided since implied in the phrase 'an event or feature may occur' is that the 'event or feature may not occur'.

Such usage gives no real guidance to the reader as to whether allowance should be made for the event or feature or not. It may very well be the case, however, that imprecision in this way is used deliberately in order to create the impression that uncertainties have been reduced to an acceptable level so that if they do occur they can be deemed as having been 'foreseeable' events. This is illusory and can serve only to form the basis for subsequent argument and dispute, for the reasons given previously.

It is to be acknowledged that the manner it which information on ground conditions for underground construction is used by Tenderers and subsequently by the appointee Contractor will be influenced by the type of contract (for example design and build, target lump- sum, admeasurement), the consequent allocation of risks within it and the ensuing mechanisms, if present, for claiming for unforeseen conditions. This will have a bearing on the nature and form in which the information is provided, but ultimately it must be in such a way that the risks can be readily identified by the various parties and due allowance made.

An interpretation of the ground conditions as included in an Interpretative Report for issue to Tenderers could differ from an interpretation used for design purposes, Ground is neither truly elastic, nor isotropic nor homogeneous and analytical modelling, no matter how apparently sophisticated, inevitably requires some form of 'simplification' of the ground, either in terms of characteristics or behaviour and consequently interpretation.

FORESEEABLE EVENTS

It will be appreciated from the foregoing that there is an inevitable foreseeability in relation to some of the situations when ground conditions are deemed by Contractors to be 'unforeseen'. These situations arise from either imprecision in terminology used in the presentation of factual data or an Interpretative Report or from the manner in which the information is presented.

Experience has shown that under such circumstances a stand-off situation arises with intransigence developing between the factions. The absence of any form of common ground on the precise meaning of even basic parameters which have been used to characterise the ground immediately creates an apparently insoluble problem. This situation can become even more desperate if there is no common ground for assessing and agreeing conditions for payment purposes.

By way of example, the Norwegian Tunnelling Index 'Q' has been used not only as a means of describing and classifying ground (and hence a means of comparing predicted against encountered) but also for payment purposes by having different rates for excavation and /or support based on 'Q' with assessed (interpreted) 'Q' values shown on contract drawings. In such cases, without clear definition and description of the derivation of the values shown and the prescriptive means of deriving values for the encountered conditions, it is foreseeable that disputes will arise over the values ascribed to the conditions being encountered. The latitude for argument arising from the subjectiveness of assessing values for each of the six parameters involved in determining 'Q', the different methods possible for determining RQD values from face-mapping, the variability of recorded ground conditions at the face and the consequent appropriate locations for scan-lines for recording discontinuity data provide an almost limitless source of discussion and debate. Whilst it is readily understandable why the third decimal place of a value for 'Q' can be so significant if it determines whether the ground conditions are 'Exceptionally Poor' or 'Extremely Poor' for payment purposes, resolution of an argument over the value can be difficult if not impossible without definition and description referred to earlier. It is also readily understandable why the Alternative Disputes Resolution procedure is potentially so useful in such circumstances - it does not apparently require resolution of the technical issues in a dispute', merely a commercial compromise!

Preventing claims for unforeseen ground conditions can be achieved by removing the mechanism, with no Clause 12 or equivalent in the Contract. The consequence of this course of action in terms of potential impact on the tendered sums is obvious.

A method to attempt to limit claims relating to ground conditions in underground construction experienced recently involved leaving in a clause relating to unforeseen conditions but writing a specification which sought to cater for 'foreseeable events' associated with the ground as encountered. With limited geological and geotechnical data available, the method-based specification not only covered the works as shown on the contract drawings but also provided the Engineer with the ability to change or modify works in progress in response to the ground conditions being encountered thereby seeking to circumvent claims for unforeseen conditions. However, by leaving a clause for unforeseen conditions in the Contract (which took precedence over the specification and drawings), the Contractor has a justifiable claim against this clause if he could demonstrate that the ground conditions whenever and wherever encountered which resulted in the Engineer changing the sequences or methods of working could not have been foreseen. In other words, general provision for foreseeable events in the specification in relation to ground conditions did not preclude such events being deemed as a consequence of unforeseen conditions when and where they actually occurred.

SUMMARY AND COMMENT

Site investigation practice has improved over the years and undoubtedly will continue to do so. The benefits to be obtained from a well planned, implemented and interpreted investigation programme can only be fully realised if suitably and appropriately qualified and experienced engineers and geologists are involved at the outset as part of the project team. The procedures recommended by the Site Investigation Steering Group under the auspices of the Institution of Civil Engineers should go a long way to rectify some of the problems that have been experienced in the past with site and ground investigations.

However, even with the best ground investigation practice there will always be a residual risk associated with unforeseen ground conditions in underground construction, because of the origin and nature of the ground. Nearly twenty years ago, the basis and justification for developing Reference Ground Conditions was put forward by CIRIA and the overall philosophy and purpose for such is considered to be as appropriate today as it was then. Experience has shown that claims for unforeseen conditions can often stem from a basic lack of communication and understanding of the terms used. There can be no excuse for this. Clarity in documentation, through definition and description, is fundamental to avoid unnecessary argument.